1. Field of the Invention
The present invention relates to a polishing pad by which optical materials such as reflecting mirrors etc., or materials requiring a high degree of surface planarity, as in the polishing of silicone wafers, glass substrates or aluminum substrates for hard disks, or general metal polishing, can be flattened with stability and high polishing efficiency. The polishing pad of the present invention can also be used particularly in a step of planarizing a silicone wafer, a device having an oxide layer, a metal layer etc. formed on a silicon wafer, or a device before lamination and formation of an oxide layer or a metal layer. Further, the present invention also relates to a method of producing the polishing pad described above.
2. Description of the Related Art
Typical materials requiring a high degree of surface planarity include a single-crystalline silicone disk called a silicone wafer for producing semiconductor integrated circuits (IC, LSI). In a process for producing IC, LSI etc., the surface of the silicone wafer should be flattened highly accurately in a step of depositing and forming an oxide layer or a metal layer in order to provide reliable semiconductor connection with various films used in manufacturing circuits. Generally, a polishing pad is stuck on a rotatable supporting disk called a platen in the polishing step, while a workpiece such as a semiconductor wafer is fixed to a polishing head. By rotational movement of the two, a relative speed is generated between the platen and the polishing head, and polishing slurry containing abrasive grains is continuously fed to the polishing pad to effect polishing.
A chemical mechanical polishing apparatus (CMP apparatus) for planarizing a processed surface by a CMP method is shown by reference to FIG. 1. The CMP apparatus used in the CMP method is provided with a polishing platen 2 for supporting a polishing pad 1 and with a supporting stand (polishing head) 4 for supporting a polished material (wafer) 3. The polishing platen 2 and the supporting stand 4 are arranged such that the polished pad 1 and the polished material 3, both of which are supported by them, are opposed to each other, and the polishing platen and the supporting stand are constituted to be capable of rotating around rotating shafts 6 and 7. The polished material 3 is stuck on the supporting stand 4 which is provided with a pressing mechanism for pushing the polished material 3 onto the polishing pad 1 at the time of polishing. An abrasive-feeding mechanism 5 is to feed an abrasive suspension having abrasive grains such as silica particles dispersed in an alkali solution to the polishing pad 1 on the polishing platen 2.
In finishing processing or in a process for multilayer interconnections for devices, a semiconductor wafer is subjected to mirror polishing by chemical mechanical polishing or polished to flatten an inter layer dielectrics and an electroconductive film. Such polishing requires polishing uniformity in the whole area of the wafer, selective polishing of protrusions in an uneven surface, and planarity after polishing of the uneven surface. For these requirements, known polishing pads constituted in the following manner have been developed and investigated.
(1) A polishing pad having a synthetic leather layer as a polishing layer laminated on an elastic polyurethane layer (U.S. Pat. No. 3,504,457).
(2) A polishing pad structured by sticking a polyurethane-impregnated nonwoven fabric on a foamed polyurethane layer (JP-A 6-21028).
(3) A polishing pad provided with a polishing surface and a rigid element of selected rigidity and thickness adjacent to the polishing surface and with an elastic element adjacent to the rigid element to endow the rigid element with substantially uniform strength, characterized in that the rigid element and the elastic element give elastic flex strength to the polishing surface to induce the controlled flex of the polishing surface so as to fit it to the whole shape of the surface of a workpiece and to maintain rigidity controlled for the local shape of the surface of the workpiece (JP-A 6-077185).
(4) A polishing cloth comprising a surface layer A having high longitudinal elastic coefficient EA and a lower layer B having low longitudinal elastic coefficient EB, characterized by being provided with a middle layer M having higher longitudinal elastic coefficient than that of the layer B between the layers A and B (JP-A 10-156724).
(5) A pad composed of a polishing layer, a middle layer having higher elasticity than that of the polishing layer, and a soft lower layer, wherein the middle layer is divided (JP-A 11-48131).
However, the polishing pads described above suffer from the following problems:
(a) For the uniformity of whole surface, the elastic polyurethane layer in the polishing pad described in U.S. Pat. No. 3,504,457 plays a role in making loading applied to a wafer uniform, and soft synthetic leather is used in the outermost polishing layer, and therefore there is no problem such as scratches, but there is the problem of poor planarizing characteristics in minute regions.
(b) In the polishing pad described in JP-A 6-21028, the nonwoven fabric layer plays the same role as that of the elastic polyurethane layer described in U.S. Pat. No. 3,504,457, to achieve uniformity. Further, the polishing layer has a rigid foamed polyurethane layer and is thus superior to the synthetic leather in planarizing characteristics, but does not reach levels required in recent years for improving planarizing characteristics in minute regions and for polishing metal films. The planarizing characteristics can be improved by further increasing the hardness of the rigid urethane layer, but in this case, scratches occur frequently, thus making this prior art pad unpractical.
(c) The polishing pad described in JP-A 6-77185 is constituted so as to have suitable hardness not causing scratches on the polishing layer as the surface layer and to permit a second rigid layer to improve planarizing characteristics deteriorated due to low hardness. This is to solve the problem in the polishing pad described in JP-A 6-21028, but in this case, the thickness of the polishing layer is limited to 0.003 inch or less, and with this thickness given, the polishing layer is also shaved to reduce the life of the product.
(d) The basic idea in the polishing pad described in JP-A 10-156724 is the same as in the art described in JP-A 6-77185, and the range of the elastic modulus of each layer is limited to achieve a more efficient range, but in this art, there is no description of any substantial realizing means, thus making production of the polishing pad difficult.
(e) The basic idea in the polishing pad described in JP-A 11-48131 is the same as in the art described in JP-A 6-77185, but the middle rigid layer is divided in a certain predetermined size to further improve uniformity in the surface of a wafer. However, the step for dividing the layer costs much, thus failing to provide an inexpensive polishing pad.
In the case of the polishing pads described in these prior art literatures, the polishing pads are generally stuck via an adhesive such as a double-coated tape on a polishing machine used, but their adhesiveness in this case is scarcely examined up to now. In the case of the polishing pad used generally and widely at present, the removal of the polishing pad from a polishing machine after use requires very large force, and the exchange of the polishing pad with a new one requires severe hard work.
The present invention provides a polishing pad used in a polishing step of planarizing a semiconductor wafer or fine unevenness of a fine pattern on a device having the pattern formed on a semiconductor wafer, wherein the polishing pad shows a high polishing rate at the time of polishing, as well as a method of polishing a semiconductor wafer. The present invention also provides a semiconductor wafer-polishing pad which in polishing a semiconductor wafer, is excellent in planarizing characteristics of a fine pattern on a device having the pattern formed on the semiconductor wafer, does not generate scratches, and can be produced at low cost, as well as a method of polishing a semiconductor wafer by using the polishing pad.
In the polishing operation, a change in dimension of the polishing pad is very important. When the polishing pad is poor in dimensional stability, a change in dimension of the polishing pad occurs due to frictional heat generated in a polishing process, which can cause deterioration in planarizing characteristics.
Conventionally, a polishing pad is often rendered highly elastic for the purpose of improving the planarity of a material to be polished, and there is no sufficient discussion on the dimensional stability of the polishing pad.
Another object of the present invention is to provide a polishing layer of a polishing pad, which undergoes less deterioration in planarizing characteristics even at an increasing temperature due to frictional heat generated in a polishing step.
As the polishing pad used in highly accurate polishing, a polyurethane foam sheet whose void volume is about 30 to 35% is generally used. Further, it is important that the polishing pad can complete polishing as rapidly as possible, and for this purpose, the polishing pad is required to have a high polishing rate.
However, when the conventional polyurethane foam sheet is used as a polishing pad or a polishing layer of a polishing pad, the polishing efficiency is improved by increasing the polishing rate, but after polishing, resistance upon removal of a wafer from the polishing pad is increased to generate dechucking error to cause problems of wafer breakage and reduction of operational efficiency.
A still other object of the present invention is to provide a polishing pad or a polishing layer of a polishing pad, which maintains a high polishing rate, exhibits less resistance upon removal of a wafer from the polishing pad after polishing to prevent generation of dechucking error, thus preventing wafer breakage and reduction of operational efficiency.
The planarity and within wafer uniformity of a polished material and polishing rate are required as the polishing characteristics of the polishing pad. The planarity and within wafer uniformity of a polished material can be improved to a certain degree by rendering the polishing layer highly elastic. The polishing rate can be improved by using cell-containing foam. However, when the polishing rate of the polishing pad during use is changed, it is necessary to regulate the polishing conditions, resulting in low polishing efficiency.
A still other object of the present invention is to provide a polishing pad excellent in planarity, within wafer uniformity and polishing rate and undergoing less change in polishing rate.
With respect to such polishing pads used in CMP, techniques described in Japanese Patent No. 3013105 and JP-A 11-322878 are known. The techniques disclosed in Japanese Patent No. 3013105 are concerned with a polishing pad comprising fine particles or hollow fine particles of water-soluble resin added to a polymer matrix such as a polyurethane polymer. The techniques disclosed in JP-A 11-322878 are concerned with a polishing pad comprising polystyrene foamed beads dispersed in a polyurethane polymer.
However, when fine particles or hollow fine particles of water-soluble resin, or polystyrene foamed beads, are dispersed in resin in the known polishing pads described above, uneven dispersion of the particles occur readily due to e.g. a difference in density from the polyurethane as matrix resin. The fine particles or hollow fine particles in the prior art described above exhibit a very important function in forming small concaves in the polishing surface of the polishing pad in CMP processing, to maintain abrasive grains on the concaves, and uneven dispersion of such particles affects the performance of the polishing pad.
On the other hand, when a polishing pad consisting of foamed polyurethane obtained by reacting and curing an isocyanate group-containing compound with an active hydrogen group-containing compound is used, sufficient polishing performance may not be obtained even if the same components are used, and thus there is demand for improvement.
A farther object of the present invention is to provide a method of producing a polishing pad having stable polishing properties with a polishing layer composed of foamed polyurethane, as well as a method of producing a polishing pad having a polishing layer whose pores are composed exclusively of polyurethane without containing fine particles or fine hollow particles of water-soluble resin, or materials such as polystyrene foamed beads for forming concaves in the polishing surface.
In the operation of polishing, heat is generated due to friction between the polishing pad and a workpiece, to increase the temperature of the surface of the polishing pad. This increase in temperature changes the hardness (modulus of elasticity) of the polishing pad, thus exerting adverse influence on planarizing processing requiring a high degree of surface planarity.
A still further object of the present invention is to provide a polishing pad capable of planarizing processing stably in a wide temperature range, which overcomes the conventional problem of the adverse influence of frictional heat between the polishing pad and a workpiece on planarizing processing due to a change in the hardness (modulus of elasticity) of the polishing pad.
As a polishing pad used in highly accurate polishing, a polishing pad (trade name IC-1000 manufactured by Rodel) comprising high-pressure-gas-containing hollow fine spheres dispersed in matrix resin such as polyurethane as described in Japanese Patent No. 3013105 is well-known. However, it cannot be said that the planarity of this polishing pad comprising high-pressure-gas-containing hollow fine spheres dispersed therein is sufficiently satisfactory. As is also described on page 17 in Detailed Semiconductor CMP Techniques (in Japanese) authored by Toshiro Doi et al. and published by Kogyo Chosakai Publishing Co., Ltd. (2000), a hard polishing pad which can further improve planarity is necessary in consideration of application to next-generation elements.
For the purpose of improving planarity, a hard polishing pad in a non-foam system (for example, IC-2000 manufactured by Rodel) can also be used. However, when such a hard pad is used, there occurs a problem of scratches on the polished surface of a workpiece, as described in the above literature (ibid.). Further, the polishing pad in a non-foam system cannot sufficiently maintain slurry abrasive grains on the surface of the pad at the time of polishing, and is thus not preferable from the viewpoint of polishing rate.
In addition, JP-A 2001-47355 proposes a polishing pad comprising a water-soluble material dispersed in a water-insoluble thermoplastic polymer. This polishing pad is a non-foam, but the water-soluble material dispersed in the polishing pad is dissolved during polishing to generate foam-like pores in the surface of the polishing pad, and the polishing pad is swollen to reduce the hardness of the surface of the polishing pad, and thus this polishing pad is significantly effective for improvement in planarity and improvement in polishing rate. However, the characteristics of this polishing pad depend greatly on the water-soluble material, and scratches caused by the thermoplastic polymer as a major component of the polishing pad cannot be solved.
A still other object of the present invention is to provide a polishing pad which can achieve improvement in planarity and reduction in scratches which cannot be simultaneously solved by the conventional polishing pads. A further object of the present invention is to provide a polishing pad which can be satisfactory from the viewpoint of polishing rate in addition to improvement in planarity and reduction in scratches.
In the polishing operation, the wetting properties of the polishing pad with polishing slurry are very important. When the polishing pad is poor in wetting properties with polishing slurry, the polishing slurry cannot sufficiently spread on the polishing pad, and in an extreme case, the polishing pad repels the polishing slurry to reduce the polishing rate significantly thus necessitating a long time for polishing. Even if the polishing slurry spreads on the polishing pad, the amount of the adhering polishing slurry is uneven, thus failing to uniformly polish an object of polishing. Further, dry regions may occur on the polishing pad, and when a small amount of polishing slurry adheres to such regions, the solution only is removed by evaporation etc., resulting in aggregation of abrasive grains on the regions, which can cause scratches as defects on the surface of a semiconductor wafer.
On the other hand, when the polishing pad is too excellent in wetting properties with polishing slurry, the solution in the polishing slurry is absorbed into the surface of the polishing pad, resulting in swelling of the polishing pad to deteriorate the stability of polishing rate.
In consideration of the instability of polishing rate caused by too excellent wetting properties, the surface of the conventional polishing pad is often rendered water-repellent. For the purpose of improving the planarity of an object of polishing, the polishing pad is often rendered highly elastic, and there was no sufficient discussion on the wetting properties of the polishing pad.
For the reason described above, it is noticed in recent years that the wetting properties of the polishing pad with polishing slurry should be improved to a certain degree, and improvements in this respect have been attempted. JP-A 2000-173958 proposes improvement in the wetting properties of the surface of a polishing pad by rendering the surface of the polishing pad hydrophilic by treatment with a chemical solution having an oxidizing action or with a surfactant etc. JP-A 2000-237951 proposes improvement in the wetting properties of the surface of a polishing cloth made of a nonwoven fabric or a polishing cloth comprising a nonwoven fabric impregnated with an elastomer, wherein the polishing cloth is rendered hydrophilic by treatment with a hydrophilic solution.
In both JP-A 2000-173958 and JP-A 2000-237951 supra, a matrix polymer material forming the polishing pad is subjected to hydrophilization treatment with some chemicals in order to improve the wetting properties of the surface of the polishing pad. Accordingly, chemicals for hydrophilization treatment are necessary in producing the polishing pad, and the step of hydrophilization treatment is also necessary, thus hardly coping with demand for reduction in costs.
A further still object of the present invention is to provide a polishing layer of a polishing pad, which has the optimum wetting properties with polishing slurry without hydrophilizing, with chemicals, a matrix polymer material forming the polishing pad (polishing layer), a process for producing the same, a polishing pad capable of polishing an object of polishing uniformly without scratches and maintaining a satisfactory polishing rate, and a process for producing the same.
As the polishing pad used in highly accurate polishing, a polyurethane foamed sheet having a void volume of about 30 to 35% is generally used. However, the polyurethane foamed sheet is excellent in topical planarizing performance, but is poor in cushioning properties due to a low compressibility of about 0.5 to 1.0%, thus hardly giving uniform pressure on the whole surface of a wafer. Accordingly, the polyurethane foamed sheet is provided on the backside thereof with a soft cushion layer and used in polishing step as a laminated polishing pad.
In the conventional polishing pad, a resin-impregnated nonwoven fabric used widely as a cushion layer has open cells through which the slurry used enters the cushion layer. As a result, the hardness, compressibility etc. of the cushion layer are changed, the characteristics of the whole polishing pad are changed, and polishing characteristics such as uniformity and polishing rate are changed with time to cause a problem of a reduction in wafer yield.
As the technique of preventing slurry from entering a resin-impregnated nonwoven fabric as the cushion layer in the conventional polishing pad, a technique of covering a side of a cushion layer with a polishing layer is disclosed in Japanese Patent No. 2842865 and a technique of arranging a waterproof material layer between polishing layers is disclosed in Japanese Patent No. 3152188.
In the technique described in Japanese Patent No. 2842865, however, the polishing layer should be in a such special shape as to cover a side of a cushion layer, and the backside of the polishing layer should form a concave so as to embed and laminate the cushion layer therein, thus significantly reducing the productivity of the polishing pad. In the technique disclosed in Japanese Patent No. 3152188, a waterproof material layer is arranged between polishing layers, and when the polishing surface approaches the waterproof material layer as the polishing layer is abraded during polishing, the difference in rigidity between the polishing layer and waterproof material layer comes to be significant, to exert adverse influence on the polishing characteristics. When the polishing layer is used in such a range that such influence is eliminated, the life of the polishing pad is reduced.
A still other object of the present invention is to provide a polishing pad with less reduction in the hardness of the cushion layer and with less change in the compressibility thereof and thus with less change in the characteristics of the whole polishing pad, thus being free of the reduction of wafer yield caused by changes with time of polishing properties such as uniformity, polishing rate etc. A still further object is to provide a polishing pad of long life having a stable and high polishing rate.